The present invention relates to spread spectrum modulation and methods therefor, and in particular to spread spectrum modulation employing multiple code generators and multiple mixers.
In certain covert communications systems it is desirable to spread a signal into a wide band that looks like noise. Such spreading can be accomplished by the use of a bi-phase modulator which splits a carrier signal into segments and alters the phase of each segment in response to a binary signal received from a pseudo-random bit sequence code generator. The response to the bit sequence is such that a logic zero level bit causes an in-phase carrier signal and a logic one level bit changes the phase of the carrier signal by 180.degree..
The greater the frequency with which bits are supplied by the binary code generator, called the code clock frequency, the greater is the spreading of signal frequency that results. Therefore, the highest possible code clock frequency is the most desirable clock frequency to employ from the standpoint of spreading the power of a signal over a range of frequencies so that the signal resembles noise. Unfortunately, high speed code generators are difficult and expensive to achieve.